@KTS915: What I meant is 'it is difficult for theme developers to find out what might be best for the users of their theme'. 90% SVG usage sounds amazing. In this case you should really have no need for adaptive images. People who do not serve many pictures might not need it, too.

Well - don`t understand me wrong. I am no advocat for the new 'fixed solution'. I see reasons enough (pro and contra) to leave things like that as a plugin (or theme development process) so that users can decide theirselves if they want to use adaptive images or not and if it makes sense in the given project. On the other hand, if it goes to core, it should be very well supported and may be upgraded later and hopefully we will even be able to turn it off if we don`t need it.